My son is 6 and my daughter is 4. I breastfed my son for about 3 or 4 months. I stopped because I was a year overdue for a mammorgram (I was scheduled for one when I found out I was pregnant) and I had to stop nursing so my milk could dry up before the mammorgram. With my daughter, I did not make enough milk for her. I would nurse, give her a bottle of formula and then pump. By the time I finished with all that it was time to start over again. After a couple of months of this, I went to formula completely.
I never gave them plain water. The only water they got was what I mixed with the formula. When they were old enough for juice, I would dilute it with water too (about half and half. I read somewhere not to give them plain water because it would fill them up and they needed the calories and nutrients in the formula. Both of my kids are healthy and right where they should be weight wise. Hope this helps.